Dry compressing vacuum pump having a gas ballast device

ABSTRACT

A dry compressing vacuum pump ( 1 ) has a continuous or graduated inner compression. A gas ballast device ( 8 ) selectively adds a ballast gas to a pumped gas. The gas ballast device has an isolating valve ( 11 ), a non-return valve ( 12 ) which prevents the escape of gases from the pump through the gas ballast device to the outside, and a pressure differential valve ( 13 ).

BACKGROUND OF THE INVENTION

The present invention relates to a dry compressing vacuum pump having acontinuous or graduated inner compression and comprising a gas ballastdevice.

The term “dry compressing vacuum pump with inner compression” denotesany vacuum pump, the pump chamber or pump chambers of which is/are freeof oil and where the volume of the pump chamber decreases in acontinuous or graduated manner from the inlet to the outlet of the pump.An example for a dry compressing vacuum pump having a continuouslydecreasing pump chamber volume is a screw vacuum pump with threads, thepitch, depth and/or width of which decrease continuously from the inletto the outlet. Examples for dry compressing vacuum pumps where the innercompression decreases in a graduated manner are multi-stage claws, Rootsor piston vacuum pumps in which the volume of the pump or compressionchambers decreases from stage to stage. Also in the instance of screwvacuum pumps it is known to design the threads such that these changetheir properties in a graduated manner.

Dry compressing vacuum pumps are generally employed in applications(semiconductor production, for example) in which toxic, very expensiveor also explosive gases need to be pumped.

It is known to employ in the instance of dry compressing vacuum pumps ofthe kind mentioned, gas ballast devices for the purpose of avoidingcondensation in the area on the outlet side. The gas ballast istherefore supplied into the pump chambers or pump chamber sections atthe outlet area.

Dry compressing vacuum pumps of the kind affected here exhibit, owing totheir inner compression in the area of their outlet, pressures which notonly exceed the inlet pressure but which can also significantly exceedatmospheric pressure. This also applies to the instance in which bypassvalves are employed, since these valves throttle a large gas flow owingto their limited cross sections. Would a vacuum pump of the kindaffected here be operated during this operational phase with an open gasballast valve, then gases pumped by the pump would enter from the pumpchamber into the atmosphere.

It is the task of the present invention to design the gas ballast devicefor a vacuum pump of the kind mentioned above in such a manner that therisk of gases escaping no longer exists. Moreover, it shall be achievedthat gas ballast operation will not impose an additional load on thepump's drive motor.

SUMMARY OF THE INVENTION

This task is solved through the characterising features of the patentclaims. In that a component of the gas ballast device is a non-returnvalve, it can be ensured that gases pumped by the pump can not escape tothe outside through the gas ballast device.

It is expedient to provide, in addition, a differential pressure valvewhich allows the admittance of the ballast gas only starting at acertain pressure difference. Through this measure it can be ensured thatthe admission of the gas ballast into the vacuum pump will only bepossible at a pressure below that defined through the differentialpressure valve. Unnecessary loading of the pump by the admitted ballastgases can thus be avoided.

Still further advantages of the present invention will become apparentto those of ordinary skill in the art upon reading and understanding thefollowing detailed description of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may take form in various components and arrangements ofcomponents, and in various steps and arrangements of steps. The drawingsare only for purposes of illustrating a preferred embodiment and are notto be construed as limiting the invention.

FIG. 1 is a schematic representation of a multi-stage pump equipped witha gas ballast device in accordance with the present invention,

FIG. 2 illustrates a specific embodiment of the gas ballast device,

FIG. 3 illustrates the rotors of a screw vacuum pump with innercompression,

FIGS. 4 and 5 illustrate examples of multistage piston vacuum pumps.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The pump 1 according to FIG. 1 comprises three stages 2, 3, 4 where thepump chamber volume decreases from the inlet 5 to the outlet 6.Connected between the last but one and the last stage is a gas ballastdevice 8 designed in accordance with the present invention which in agas ballast feed line 9 has—arranged in any sequence—an isolating valve11, a non-return valve 12 and a differential pressure valve 13.

With the aid of the isolating valve 11, the gas ballast mode may beswitched on and off in a known manner. The non-return valve 12 is sobuilt-in that it prevents gases being pumped by the pump 1 from escapingthrough the line 9. The differential pressure valve 13 has the effectthat with valve 11 open, the ballast gas can only enter into the pump 1when the pressure in the area of the gas ballast inlet drops below apressure defined by the differential pressure valve.

Depicted in FIG. 2 is an embodiment for the gas ballast device 8 whichis affixed directly to the casing 15 of a vacuum pump 1. Said gasballast device comprises the casing 16, which is affixed to the vacuumpump 1 with the aid of a bolt 17. Bolt 17 is screwed into the channel 18serving the purpose of feeding the ballast gas in, and said screw has inthis area a hollow shank 19 which is linked via a lateral opening 21 tothe inside chamber 22 of the casing 16. Located in the hollow chamber 19is the non-return valve 12. It consists of a ball 23 (made of anelastomer material, for example), a seat 24 (made of steel, for example)and a spring 25 working in the direction of the closed position.

The inside chamber 22 of the cylindrically designed casing 16 haslateral openings 27. A rotatable sleeve 28 encompassing the casing 16has, in the position shown, concentric cut-outs 29 with respect to theopenings 27. The gas ballast feed is opened and closed by turning thesleeve 28.

Depicted in FIG. 3 are the rotors 31, 32 of a dry compressing vacuumpump 1 operating in accordance with the screw principle. Inlet andoutlet are depicted schematically through arrows 34, 35. The threads ofthe rotors 31, 32 decrease in pitch, and the land of the threads alsodecreases. In the vicinity of the outlet there is provided a gas ballastfeed through the gas ballast device 8.

Screw vacuum pumps are expediently operated with a significant amount ofinner compression so that a maximum power uptake of the drive motor isattained at an inlet pressure of approximately 300 mbar. At this inletpressure no ballast gas needs to be fed in, since the then commonly hightemperatures of the pump prevent any condensation. If in this operatingmode ballast gas were to be pumped, this would result in an additionaluptake of power, i.e. additional motor power would have to be available.For this reason it is expedient to rate the differential pressure valve13 so that the gas ballast supply can only be effected at a relativelygreat pressure difference. If, for example, the opening pressure of thedifferential pressure valve amounts to 900 mbar, the gas ballast couldthen only be admitted at a pressure of about 100 mbar (atmosphericpressure minus 900 mbar). In this operating mode, full motor power is nolonger required so that no higher motor power needs to be installed forthe gas ballast.

Depicted in FIGS. 4 and 5 is an embodiment (only partly in FIG. 4) of adry compressing vacuum pump which is designed by way of a multi-stagepiston vacuum pump. Located in its pump chamber casing sections 41 and42 are the cylindrically shaped pump chambers 43 to 46. Located betweencasing section 41, 42 is the crankshaft chamber 47, the casing of whichis designated as 48. The pistons 51 to 54 are each graduated and formeight pump chambers which are in part connected in parallel so that thedepicted pump has four pumping stages decreasing in volume. Its inlet isdesigned as 55 and its outlet as 56. In the older German patentapplication 196 34 519.7 a vacuum pump of this kind is individuallydetailed. The last ring-shaped pump chamber forms the last stage of thevacuum pump depicted. Its inlet is designated as 57, its outlet as 58.

In the embodiment in accordance with FIG. 4, the gas ballast is suppliedinto the connecting line between the outlet of the last but one pumpingstage and the inlet 57 of the last pumping stage. Gas ballast device 8is connected to this connecting line.

In the embodiment with FIG. 5, the gas ballast is supplied through thecrankshaft chamber 47 as is basically known from U.S. Pat. No.6,123,516.

The inlet 57 of the last stage of the pump is linked via line 59 to thecrankshaft chamber 47. The opening of said line forms the gas ballastinlet 61 in the vicinity of the pump chamber. Said opening is located inthe vicinity of one of the face sides of crankshaft casing 48. In thearea of the side opposing the crankshaft casing 48, there is located thegas ballast or purge gas inlet 8. With gas flowing in through the gasinlet 8, the crankshaft chamber 47 may be purged and/or an overpressurecan be maintained within.

In the instance of piston vacuum pumps it is important that the pressurein the crankshaft casing 47 matches the pressure in the pump chambers.In particular starting a piston vacuum pump in the presence of a highpressure (atmospheric pressure, for example) in the crankshaft casingand a vacuum in the pump chambers 43 to 46 is difficult when employingAC motors offering only a weak starting torque. This will be the casewhen the pump is shut down with the vacuum chamber evacuated whilepurging the crankshaft casing 47 via the opened gas ballast device 8.If, however, the gas ballast supply is only opened after a pressuredifference has been exceeded, a low pressure can be maintained in thecrankshaft casing also when shutting the pump down. If, for example, thepressure difference defined by the differential pressure valve is 600mbar, then with the gas ballast device 8 open, the crankshaft casing 47will only be vented up to a pressure of approximately 400 mbar(atmospheric pressure minus 600 mbar).

The invention has been described with reference to the preferredembodiment. Obviously, modifications and alterations will occur toothers upon reading and understanding the preceding detaileddescription. It is intended that the invention be construed as includingall such modifications and alterations insofar as they come within thescope of the appended claims or the equivalents thereof.

Having thus described the preferred embodiment, the invention is nowclaimed to be:
 1. A dry compressing vacuum pump comprising: a continuousor graduated inner compression chamber; pumping element movably disposedin the compression chamber; a gas ballast device connected with thechamber and having connected in series: (1) an isolating valve; (2) anon-return valve preventing the escape of gases from the pump throughthe gas ballast device to the outside; and a differential pressure valvewhich attains its open position only when a difference betweenatmospheric pressure and a pressure present at the differential pressurevalve on the pump side exceeds 500 mbar.
 2. The pump according to claim1, wherein the vacuum pump is a screw vacuum pump and comprising pluralpumping elements which include inter-engaging screw elements.
 3. Thepump according to claim 2, wherein the differential pressure valve opensat a pressure difference of 800 to 1000 mbar.
 4. The pump according toclaim 1, wherein the vacuum pump is a multi-stage piston vacuum pumpwhich has a plurality of compression chambers, each with a pumpingelement.
 5. The pump according to claim 4, further including: a jointcrankshaft chamber, the ballast gas being supplied through thecrankshaft chamber.
 6. The pump according to claim 4, wherein the gasballast device is connected to a line which links the outlet of apenultimate stage to an inlet of the last stage.
 7. A multi-state vacuumpump comprising: a plurality of compression chambers interconnected todefine a plurality of pumping stages; a pumping element movably mountedin each chamber; a gas ballast device having: an interconnectedisolating valve and a non-return valve connected with one of the stagesto prevent the escape of gases from the pump through the gas ballastdevice to the outside, and a differential pressure valve configured toopen at a pressure difference of 500 to 1000 mbar to admit gas from theoutside to pass through the gas ballast device into the one stage.
 8. Acompressing vacuum pump comprising: a plurality of compression stageswith progressively higher compression from a first stage to a laststage; a gas ballast which selectively supplies a ballast gas to one ofthe stages after the first stage and before the last stage, the gasballast including: a one-way valve and a pressure differential valvewhich permit the ballast gas to enter the one-stage when pressure in theone stage is below a pressure of the ballast gas by a 500-1000 mbarpressure differential and which block gases from leaving the one stage.